Smallpox vaccine could result in deaths and serious side effects

New York TimesBy NICHOLAS BAKALAR

A significant number of deaths and serious side effects could result from a mass smallpox vaccination program, according to a newly published analysis of historical data, and the exact numbers might depend on the strain of virus used to make the vaccine.

The smallpox vaccine is a pure preparation of live vaccinia virus, a germ similar to cowpox and smallpox, and there are several different types. The United States stockpiles a strain called New York City Board of Health, or N.Y.C.B.H., while most European countries have stocks of the Lister strain.

Complications from the smallpox vaccine are not common, and deaths are rare. But of the two strains generally stored today, N.Y.C.B.H. is considerably safer than Lister, researchers say. While new and presumably safer vaccines are under development, there is no way to know their side effects until they have been used in a large vaccination campaign.

The study appears online in the August issue of PLoS Medicine.

In 1968, the World Health Organization recommended that either the Lister or the New York strain be used exclusively for the worldwide eradication campaign that ended in 1979.

“People in the U.S. can count themselves lucky that they live where a less pathogenic strain of vaccinia is used,” said Mirjam Kretzschmar, the lead author of the study. The Lister strain, she said, is still used in Europe only because the stockpiled virus was easily available as seed virus for new vaccine production.

Smallpox vaccines can cause serious but nonfatal skin and eye infections, but in their study the researchers concentrated on two specific side effects: a swelling of the brain called postvaccinal encephalitis, and death.

Older strains were more dangerous. For encephalitis, the Bern strain was the most hazardous, with 44.9 cases per one million vaccinations, followed by the Copenhagen strain with 33.3 cases.

The Lister strain caused 26.2 cases per million, and the New York strain was safest, at 2.9 cases of encephalitis per million vaccinations.

The pattern for mortality was similar. The Bern strain caused 55 deaths per million vaccinations; Copenhagen, 31.2 deaths; and Lister, 8.4 deaths. The New York strain led to 1.4 deaths per million vaccinations.

A small number of people will die in mass vaccinations no matter which strain is used. In the Netherlands, for example, which has a population of 16 million, the researchers estimate that 10 people would die in a mass vaccination program that used the New York strain, and 55 if the Lister strain was used.

Still, even the highest death rates from vaccination would be dwarfed by the number of people who could die in a widespread smallpox epidemic. While most smallpox patients recover, death can occur in up to a third of cases.

The study emphasized that mortality data from the historical record of vaccine use is not completely reliable and leaves considerable doubt about exactly what would happen in a mass vaccination program undertaken today.

The recent campaign in the United States to immunize military personnel, which resulted in fewer adverse events than in the past, gives some suggestion about current risks. But the target population for the program was younger and healthier than the general population, and it is difficult to apply those results to a large number of people.

For the New York strain, the frequency of encephalitis and mortality was highest among babies less than a year old. It decreased for children ages 1 to 3, and increased after that.

The New York City Department of Health recommends that people caring for infants under a year old not be vaccinated, because vaccinia can be transmitted by close contact. A weakened immune system, pregnancy and a history of skin diseases like eczema are also contraindications for vaccination, according to the department.

According to the Centers for Disease Control and Prevention, the federal government now has enough vaccine for every person in the United States.